1. Field of the Invention
The present invention relates to the treatment of waste gases generally and, more particularly, to a novel process for the removal of nitrogen oxides and sulfur oxides from such gases using chloric acid as an oxidizer.
2. Background Art
The amount of NO.sub.x and SO.sub.x in gases released, for example, from chemical operations, manufacturing facilities and power plants is limited by environmental air discharge permits issued by local, state, federal and regulatory agencies worldwide. Air discharge permits, in many locations in the United States, are being reviewed and revised to include all of the NO.sub.x and SO.sub.x gaseous species emitted. In many cases, the allowable NO.sub.x gas discharge in a waste gas stream can be limited to concentrations as low as about 10 to 1.00 ppm in the output gas stream.
The NO.sub.x or gaseous nitrogen oxides in waste gases are generally composed of a mixture of NO, NO.sub.2, N.sub.2 O.sub.3, N.sub.2 O.sub.4, and N.sub.2 O.sub.5. As the gas phase generally contains water vapor, oxyacids such as HNO.sub.2 and HNO.sub.3 can also be present in the NO.sub.x gas stream. In addition, there are processes which can generate significant amounts of N.sub.2 O (nitrous oxide). Except for NO, nitric oxide, most of the other nitrogen oxides can be easily removed or scrubbed from the waste gas using suitable concentrations of alkaline solutions such as dilute NaOH, carbonates, or those containing aqueous sulfur-based reducing compounds such as sodium sulfide (Na.sub.2 S).
The removal of NO.sub.x by scrubbing is particularly a problem when the waste gas contains substantial quantities of NO in the NO.sub.x gas stream. In this case, NO must be oxidized to NO.sub.2 prior to removal by conventional scrubbing using water or alkaline chemical solutions.
The deleterious effects of sulfur oxides (SO.sub.x) include the formation of acid rain (H.sub.2 SO.sub.4) by the reaction of sulfur oxides with atmospheric moisture. Acid rain has a pH of 2 or less and is responsible for acidifying streams and lakes. In addition, the presence of gaseous sulfur oxides may contribute to respiratory problems, to reduced atmospheric visibility, and to corrosion of materials.
Various oxidizing agents have been used in processes for removing NO.sub.x and SO.sub.x oxides from gaseous streams. Sodium chlorite and dilute solutions of chlorine dioxide (ClO.sub.2) have been found to be very efficient in NO oxidation. Sodium hypochlorite, potassium permanganate, hydrogen peroxide, and other peroxyacid based oxidizers are significantly less efficient in NO oxidation. Ozone (O.sub.3) is an effficient NO oxidation agent, but it is expensive to generate.
The use of chlorous acid for the removal of sulfur dioxide from combustion gases has been described in U.S. Pat. No. 4,968,501, issued Nov. 6, 1990 to J. A. Mason. The chlorous acid is formed by reacting a solution of sodium chlorite with an organic acid such as lactic, citric, malic, tartaric or glycolic acids. The chlorous acid is further reacted to form chlorine dioxide which reacts with the SO.sub.2 present in the gas stream. Chloric acid is produced as an undesirable by-product during the formation of the chlorous acid.
A SO.sub.x and NO.sub.x -containing waste gas is oxidized with an oxidizing solution containing KClO.sub.3 or NaClO.sub.3 to oxidize NO to NO.sub.2, then the treated gas is scrubbed with an alkaline absorbent to absorb NO.sub.x and SO.sub.x ( Japanese patent no. 76-95977, issued Aug. 23, 1976 to T. Matsuda et al. Chem. Abstracts 87(4):28282p).
Japanese Patent Office, Patent Disclosure No. 74-113,598, published Apr. 2, 1976 by A. Kita, describes a process for removing nitrogen oxide from waste gases by contacting the gas with an aqueous solution containing an alkali metal or alkaline earth metal chlorate and an alkali metal or alkaline earth metal chloride. The NO is almost completely oxidized to NO.sub.2, after which the gas is treated with a conventional denitrating agent i.e., Na.sub.2 SO.sub.3 (Chem. Abstracts 85(24): 181722k).
Hydrogen sulfide in fluid streams is removed upon contact with a substantially neutral aqueous stream containing a polyvalent metal chelate. An oxidizing agent is used to oxidize elemental sulfur to soluble sulfur compounds with SO.sub.2 being preferred. Other oxidizing agents which can be used include the alkali metal salts of inorganic oxidizing agents such as perchloric, chloric, hypochlorous and permanganic acids. (U.S. Pat. No. 4,451,442, issued May 29, 1984 to G. C. Jeffrey et al).
Y. Yamashita et al teach a process for the removal of NO.sub.x from waste gases using an aqueous solution of ClO.sub.2 (pH 5-8) in the first scrubber followed by a second scrubber containing Na.sub.2 S. (Kyowa Chem. Indus. Japanese Patent Office, Patent Disclosure No. 1990-169012, Jun. 29, 1990).
Mixtures of alkali metal chlorates or alkaline earth metal chlorates with sulfuric acid have been used to remove NO.sub.x and SO.sub.x oxides from a waste gas (boiler flue gas, combustion gas). The gas is contacted with a solution containing Na or Ca chlorate, H.sub.2 SO.sub.4 and a ferrous salt to oxidize NO to NO.sub.2. The solution amy also contain HC1 or a chloride which produces HCl. (Sumitomo Chemical Co. Japan. Publn. 51-20772 Feb. 19, 1976). Similarly, a SO.sub.x and NOx-contg. flue gas is oxidized with a solution containing K or Na chlorate and H.sub.2 SO.sub.4 to oxidize NO to NO.sub.2. The treated gas is then contacted with a red mud slurry obtained during alumina production. (Sumitomo Chemical Co. Japan. Publn. 53-113761, Oct. 4, 1978). However, the Applicants have found that the presence of sulfate ions in the chlorate-H.sub.2 SO.sub.4 mixture hinders or reduces the oxidation of NO gas resulting in lower NO gas removal efficiency.
There is need for a process of removing NO.sub.x and SO.sub.x oxides from gaseous streams which does not result in the formation of liquid effluents containing salts of inorganic acids. Further, there is need for an improved process of oxidizing NO and SO.sub.2 present in gaseous waste streams.